As it is known, there is a movement sensor of the inductive type which uses the LVDT principles, which consist of a primary coil and two secondary coils with a common movable magnetic core.
Sensors of the LVDT type are composed substantially of a fixed part and a movable part, both of which must be anchored to the two ends of the spring, or in any case to two separate points thereof; the measurement of the elongation is determined indirectly by the measurement of the relative position of the two parts of the sensor.
With a similar technique it is also possible to provide capacitive sensors.
Other sensors which can be used for the purpose are load cells, which measure the load to which the spring is subjected and allow to determine, starting from such measurement, the extent of the elongation.
In order to be able to measure the force applied by the spring, the cell must be connected between a fixed point and one end of the spring, or between the two ends of the spring.
These kinds of sensor are not free from drawbacks, including the fact that although they can be applied to the measurement of the elongation of a spring, they necessarily require the use of two anchoring points, at least one of which belongs to the body of the spring.